April, 1904.] Galls and Insects Producing Them. 125 



Pachypsylla celtidis mamma Riley (Figs. looa, b), and P. c. -gemma 

 Riley. 



The stud}^ of these mouthparts gave no new anatomical facts. 

 The different genera showed considerable variation as to length 

 of beak and setae. In general it may be said that the setae tend 

 to increase in the distance the}' may be protruded beyond the tip 

 of the beak as the galls approach complexity. This, however, 

 cannot be considered an exact rule, since the S. americana, C. 

 ulmicola and H. hamamelis have setae of practically the same 

 length, although the gall produced by S. americana is much 

 simpler than the galls produced by either C. ulmicola and H. 

 hamamelis (Part I, Figs. 12, 13 and 15). It was impossible to 

 make exact measurements of the distance the setae protruded 

 beyond the tip of the beak, since it was impossible to tell whether 

 the setae were fully extended or partially retracted. The above 

 conclusions were reached after the examination of a large number 

 of specimens. 



So far as I have been able to determine the insects do not 

 remain attached to any one point for a great length of time. The 

 P. c. -mamma (Figs. looa, b) has a gall of the greatest complex- 

 ity, and the insect has setae which protrude farther beyond the 

 point of the beak than any other examined ; a large number of 

 these galls were opened and the position of the insect noted. The 

 insect was never found attached and apparently had no definite 

 point of attack. 



The preceding observations emphasize Conclusions 6 and 8 of 

 Part I and a statement in the first of Part V. That is, the modi- 

 fication of the plant tissue to form the gall is purely mechanical, 

 being a continuous effort on the part of the plant to heal the 

 wound produced by the repeated puncturing of the cells by the 

 insect. When a branch is cut from a tree a growth is produced 

 which tends to cover the wound. In this case a single wound 

 and a single stimulus which is purely mechanical but which pro- 

 duces rapid growth for the purpose of covering the wound. In 

 the case of Aphididae and the Psyllidae galls the wounds are 

 more slight but repeated rapidly, the stimulus is mechanical and 

 the growth rapid, tending to cover the injury. 



It is possible that the setae of the various genera may stimulate 

 different tissues and thus cause galls of varying complexity, but 

 upon this question I am not ready to give a definite statement. 



2. DIPTERA. 



The Cecidomyid galls occur upon a greater variety of hosts than 

 any other group of galls, and as previously stated in Part V, show 

 by far the greatest variation in structural characters and the 

 smallest number of typical characters. 



